Cartridges adapted to be used with such a motorized syringe are made of transparent glass and have dental anesthetic solution filled therein. There are two types of cartridges containing 1.8 ml and 1.0 ml of anesthetic solution, and due to the fact that the cartridges have the same inside and outside diameters, they are different in length. In the case of either cartridge, upon administrating anesthetic, a cartridge holder is connected to the motorized syringe by means of a coupling, the cartridge holder having the cartridge loaded therein, and a double pointed needle is attached to the forward end of the cartridge holder to bring it into communication with the interior of the cartridge.
The motorized syringe includes a push rod adapted to push a plunger rubber plug in the cartridge to deliver the anesthetic solution from the cartridge through the needle, and an operating device for moving the push rod forward. Such an operating device generally includes a pinion rotated through a power transmission from a drive unit including a motor and a reduction gear, and a rack formed on the push rod and engaging the rotating pinion to linearly move the push rod. In order to deliver the anesthetic solution from the cartridge loaded in the cartridge holder, the linear movement of the push rod must be performed for a distance equal to the length of accommodation of anesthetic solution in the cartridge. The distance of linear movement of the push rod is defined by bringing a limit switch into an off-state with a particular portion of the push rod to stop the rotation of the motor. After delivery of anesthetic solution, the cartridge holder is detached from the coupling, and the power transmission from the drive unit is interrupted to bring the pinion into an idle state. The push rod can thus be pushed into the syringe body by hand. In this process, the limit switch is brought into an on-state with the outer surface of the push rod.
A cartridge-type motorized syringe, as well as a cartridge holder and a push rod, which the applicant is presently manufacturing and selling to dentists, is arranged as a syringe for exclusive use of each of a 1.0 ml cartridge and a 1.8 ml cartridge since the distances required to push plunger rubber plugs in both the cartridges are different due to the difference in length between the 1.0 ml cartridge and the 1.8 ml cartridge. With the spread of motorized syringes, the appearance of a new motorized syringe capable of being used in common for both 1.0 ml and 1.8 ml cartridges has been desired by dentists. In order to realize their desire, bearing the arrangement of the conventional motorized syringe in mind, it is required to use a cartridge holder in common for both 1.0 ml cartridges and 1.8 ml cartridges, to establish a common terminal position of a push rod, with respect to 1.0 ml cartridges and 1.8 ml cartridges, and to draw back a start position of the push rod for the 1.8 ml cartridge by a distance equal to the difference in length between the 1.0 ml and 1.8 ml cartridges. Since the push rod has a length required to push a plunger rubber plug throughout the length of the 1.8 ml cartridge, it must extend rearward by a distance equal to the difference in length between the cartridges. For this reason, the motorized syringe body is compelled to have a rearward protrusive configuration. The invention made on the basis of this idea is disclosed in patent reference 1. This patent reference 1 also discloses that in either cartridge-setting position, a push rod is urged against a plunger rubber plug in a cartridge under a constant pressure. However, the invention disclosed in patent reference 1 is undesirable because it does not make a configuration of the motorized syringe compact.
Patent reference 1: U.S. Pat. No. 4,198,435